I. Field of the Invention
The present invention relates to apparatus and methods pertaining to the encoding and decoding of a television signal in a manner which deters unauthorized utilization of the signal.
II. Description of the Prior Art
Present day subscription television systems are based on the generally accepted fact that rendering a video picture unviewable in a normal television receiver is most easily and effectively accomplished by simply upsetting the horizontal synchronization of the receiver. It is known that horizontal synchronization can effectively be upset for standard NTSC receivers by simply removing the horizontal synchronization pulses or by suppressing the horizontal synchronization pulses below the average video level of the signal. This causes the television receiver to try to lock horizontally on random video peaks transmitted during the active or video trace line portions of the video signal rather than on the actual horizontal synchronization pulses. The fact that the horizontal synchronization pulses are no longer consistently processed also destroys a receiver's ability to use a color reference burst associated with the horizontal synchronization signals, thereby causing inaccurate color reproduction.
However, known systems which employ removal or suppression of horizontal synchronization pulses, require employment of an encoding signal, such as a control code or keying signal, to allow a decoder to reconstruct the missing or suppressed horizontal synchronization pulses. For example, the control code or keying signal may take the form of negative going encoding pulses inserted in selected locations of the horizontal blanking pulses. In the alternative, a totally independent signal, which contains the critical encoding information needed to reconstruct the horizontal synchronization pulses may be sent by a separate communication medium, such as the audio portion of the television signal or a telephone land line.
In addition to having the disadvantage of requiring transmission of an encoding signal to reconstruct the missing or suppressed horizontal synchronization pulses, presently known systems, which employ horizontal synchronization pulse suppression or removal, generate pictures which during a sequence with a stationary vertical (gray or black) line will sometimes allow a normal receiver to horizontally lock and display a recognizable picture, albeit with severe color distortion.
Some known subscription television systems employ circuitry to invert all or selected portions of the video signal to prevent unauthorized demodulation of the video signal content. For example, systems have been described which transmit even number fields with a conventional polarity and transmit odd number fields with reverse polarity with the resultant video signal generating a blank or washed-out picture in a standard television receiver. Other systems randomly invert fields or individual lines of video to establish a scrambled signal. To decode randomly inverted video, as is the case with decoding missing or suppressed horizontal synch pulses, known systems generally require the generation and communication of an additional control code, keying signal, or encoding signal, to allow a decoder to correctly reinvert the randomly inverted portions of the scrambled signal.
While systems have been described which employ aspects of both horizontal synchronization modification and video signal polarity modification to generate a scrambled signal, such systems also require the further inefficient process of generating, communicating, and decoding of some form of encoding signal in addition to the scrambled video signal.
It is therefore an object of the present invention to economically and efficiently render the video portion of the television signal scrambled to the point of being effectively unwatchable when displayed on a standard television receiver.
Another object of the present invention is to provide a subscription television system in which the scrambling of the video signal is secure in that it becomes difficult and costly for an unauthorized viewer to reconstruct to an acceptable viewing level.
A further object of the present invention is to provide a scrambling system having encoding and decoding devices which are economical to manufacture and install and yet provide a high quality video picture with a minimum of distortion and a minimum of viewer awareness that the picture is reconstructed from a previously scrambled transmission.
More specifically, it is an object of the present invention to provide a subscription television system wherein no complex control codes, keys or encoding signals are transmitted to control operation of a decoder, but rather the system employs a decoder which operates without preestablished or encoder-controlled format to follow the transmission sequence of suppressed horizontal blanking pulses and inverted video signals as they occur.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.